Stress distribution characteristics in dental implant influenced by its wall thickness

Transcription

1 Stress distribution characteristics in dental implant influenced b its wall thickness R.C. van Staden, H. Guan & Y.C. Loo Griffith School of Engineering, Griffith Universit, Gold Coast Campus, Queensland 4222, Australia N.W. Johnson School of Dentistr and Oral Health, Griffith Universit, Gold Coast Campus, Queensland 4222, Australia N. Meredith Neoss Pt Ltd, Harrogate HG1 2PW, United Kingdom ABSTRACT: This paper aims to evaluate the stress characteristics within the dental implant influenced b different implant wall thicknesses as a result of varing diameters of 3.5 and 5.5mm. A two-dimensional finite element model of the implant and mandibular bone consisting of triangular and quadrilateral plane strain elements is analsed to compute the von Mises stresses in the implant subjected to varied masticator forces and abutment screw preloads. As epected the implant wall thickness significantl influences the stress magnitude and distribution pattern within the implant. Stress concentrations generall occur at the top of the implant as well as around the notch. When the wall thickness is reduced, stress concentrations are also found around the first eternal implant thread. The results also show that the masticator force is more influential on the stress within the implant than the abutment screw preload. 1 INTRODUCTION The modern dental implant is a biocompatible titanium device surgicall placed into the jawbone to support a prosthetic tooth crown in order to replace missing teeth. The establishment of a strong biomechanical bond between implant and jawbone is called osseointegration (Branemark et al. 1969, 1977). The success or failure of osseointegration depends on man factors including mainl: medical status of the patient, smoking habits, bone qualit, bacterial contamination, immediate loading and implant surface characteristics (Esposito et al. 1998). For both earl and late implant failures, loading is considered an important factor (Geng et al. 2001). The stress characteristics within the implant are influenced b the implant dimensions, as documented b Huang et al. (2005), Capodiferro et al. (2006) and Tolman and Lane (1992). Catastrophic mechanical failure of the implant ma occur b implant fatigue (Huang et al. 2005, Capodiferro et al. 2006), implant fractures, veneering resin/ceramic fractures or other mechanical retention failures (Winklere et al. 2003, Naert et al. 1992, Tolman and Lane 1992). From an engineering perspective, an important criterion in designing an implant is to have a geometr that can minimie mechanical failure caused b an etensive range of loading. van Staden et al. (2007) have shown that with reduced implant diameters the risk of failure is increased. As part of the implantation process, the torque is applied to the abutment screw causing an equivalent preload or clamping force between the abutment and implant. This is to ensure that the various implant components are perfectl attached to each other. However, to date no published research appears to have investigated the influence of both masticator forces and abutment screw preloading on stresses in implants influenced b the implant wall thickness due to various implant diameters. Therefore, the aim of this stud is to evaluate the stresses within an immediatel loaded implant under a range of masticator forces, abutment preloads and implant diameters.

2 Mandible 2D Slice F M 2D Slice 45 o - a) Top view b) Isometric view Figure 1. Location of 2D slice in a mandible. F M Masticator force, F M = 200, 500, 1000N VV 1 Crown F M 45 o (0-1.51mm) ( mm) ( mm) VV 2 VV 3 F P F P Abutment Abutment screw torque, T = 110, 320, 550Nmm (Equivalent abutment screw preload, F P = , , N) Implant (Diameter, D = 3.5, 5.5mm) (Length, L = 11mm) Cancellous bone (Young s modulus = 1GPa) Cortical bone (Young s modulus = 13.7GPa) (Thickness = 1.2mm) VV 4 Fied constraints Figure 2. Finite element model of implant, components, implant/bone interface and bone. 2 METHODOLOGY 2.1 Modelling A cross-sectional slice is taken from the mandible as shown in Figure 1. The arc length of the mandible is comparable to the width and depth of the slice. When the slice is subjected to in plane (-) masticator forces (F M ), it is restraint from deforming out-of-plane (in the -ais). Therefore, it can be assumed that all the strains are confined in the -ais. To accuratel represent the mechanical behavior of the implant and bone, 3-node triangular (Tri3) and 4-node quadrilateral (Quad4) plane strain elements are therefore used for the construction of the finite element models of the implant/bone sstem. This is presented in Figure 2. The two-dimensional (2D) model is analsed using Strand7 (2004) Finite Element Analsis (FEA) sstem. Data is acquired for the bone dimensions b computed tomograph (CT) scanning. Two different tpes of bone, cancellous and cortical, are distinguished and the boundaries are identified in order to assign different material properties within the finite element model. 3.5mm 5.5mm Implant wall thickness (mm) Figure 3. Finite element model showing different implant diameters. The implant is based on that manufactured b Neoss (2006), which is conical with 2 degrees of taper and has a helical thread. Two different implant diameters (3.5 and 5.5mm) as well as the corresponding wall thicknesses are shown in Figure 3. Note that 4.0 and 4.5mm diameter implants are also popularl used and

3 detailed studies on all four diameters can be found elsewhere (van Staden et al. 2007). The material properties adopted are specified in terms of Young s modulus, Poisson s ratio and densit for the implant and all associated components (Table 1). All materials are assumed to ehibit linear, homogeneous elastic behavior. For the finite element model with D = 3.5mm, L = 11mm, Tcor = 1.2mm, 3314 plate elements and 3665 nodal points are used for the implant, 3804 elements and 4079 nodes for cancellous bone, and 1216 elements and 1453 nodes for cortical bone. Table 1. Material properties Young s Poisson s Densit, Component modulus, E ratio, v ρ (GPa) (g/cm 3 ) Implant, abutment, washer Abutment screw Crown Cancellous bone Cortical bone The loading and restraint conditions as well as the detailed parameters are also illustrated in Figure 2. To evaluate the effect on stress characteristics in the implant due to varing wall thicknesses, three different mastication forces (F M ) and three different abutment preloads (F P ) are considered. The techniques for appling F M to the crown and F P to the abutment screw are discussed respectivel in Sections 2.2 and 2.3. Note that the outer surface of the jawbone is restrained in the, and directions. Note also that complete osseointegration at the implant/bone interface is assumed. As indicated in Figure 2 the von Mises stresses along the lines (0-1.51mm in length), ( mm) and ( mm) are measured for all possible combinations of loading and diameters. These lines are approimatel centre lines of the implant wall. Each line is identified b its start and end points, for eample, line begins at VV 1 and ends at VV 2. These locations were suggested b clinicians to be prone to micro fractures. 2.2 Masticator force, F M During masticator function the occlusal surface of the crown is subjected to oblique loads applied at 45 o inclination in the - plane (refer to Figure 2). The present stud assumes that the masticator force, F M, is set at 200, 500 and 1000N, as shown in Figure 2. The theoretical stud b Choi et al. (2005) suggests that this loading condition can be considered to represent realistic loading functions. 2.3 Preload, F P The torque applied to the abutment screw causes the preload or the clamping force between the implant and abutment. The procedure for calculating F P (or torque) and appling the equivalent temperature sensitive elements throughout the abutment screw is described below. An implant sstem tpicall consists of an implant, crown, abutment and abutment screw. The method for assembl of implants involves surgicall placing (or screwing) the implant into the bone. Subsequentl the abutment is attached b an abutment screw, which is mechanicall screwed into the internal thread of the implant using a manual screwdriver. Finall, the crown is placed on to the abutment using cement at the crown and abutment interface. Neoss (2006) recommends that the abutment screw is tightened b a torque of 200Nmm. Note that in this stud three levels of torque are considered to cover the situation of under (i.e. 110Nmm), average (i.e. 320Nmm), and over tightening (i.e. 550Nmm). The equivalent preload F P, as a result of the torque, can be calculated through the procedure outlined below. The nature of the forces clamping implant components together, and how the are generated and sustained, are not comprehensivel covered in the literature. Preload was considered in the finite element model b Haack et al. (1995), Lang et al. (2003) and Brne et al. (2006). These studies were based on complicated three-dimensional (3D) modeling and the techniques used for replicating F P were inadequatel specified. The relationship between the torque applied to the abutment screw, T and the equivalent preload, F P was suggested b Dekker (1995) as: T i μt rt F + + μ nr 2π cos β = P n (1) where T = torque applied to the abutment screw (Nmm) = 320Nmm (Neoss 2006); F P = preload created in the abutment screw (N) (to be determined); i = pitch of the abutment screw threads (mm) = 0.40mm (Neoss 2006); μ t = coefficient of friction between abutment screw and internal implant screw thread surfaces (dimensionless) = 0.26 (Lang et al. 2003); r t = effective contact radius between the inner implant and the abutment screw threads (mm) = (r 3 + r 4 ) / 2= ( ) / 2 = 0.87mm (Neoss 2006) (see Figure 4); β = half-angle of the threads (degree)

4 = o (Neoss 2006); μ n = coefficient of friction between the face of the abutment screw and the upper surface of the abutment (dimensionless) = 0.20 (Lang et al. 2003); and r n = effective radius of contact between the abutment and implant surface (mm) = (r 1 + r 2 ) / 2 = ( ) / 2 = 1.11mm (Neoss 2006) (see Figure 4). Note that the effective radii (r t and r n ) are the distances between the geometric centre of the part (implant, abutment or abutment screw) and the circle of points through which the resultant contact forces between mating parts (implant, abutment or abutment screw) pass (refer to Figure 4). Equation 1 then ields F P = N. The preload clamping the abutment to the implant is transferred through two interfaces. The first interface (SA 1 ) is between the abutment and abutment screw and the second (SA 2 ), between the abutment screw threads and inner threads of the implant (refer to Figure 4). The calculated preload, F P, is assumed to act equall, as a pressure, q, across the first and second interfaces. Due to equilibrium, onl the pressure q, acting on SA 1 is considered in this stud. SA 1 = (π r 1 2 ) - (π r 2 2 ) = (π ) - (π ) = 2.27mm 2 The pressure acting on SA 1, when F P = N, is calculated as follows: FP q = = = N/mm 2 (2) SA The clamping pressure, q, is a result of the applied torque and is a means of replicating the 3D torque in a 2D manner. For the present stud, q is calculated as above for the applied torques of 110, 320 and 550Nmm (Table 2). To produce equivalent q or F P, temperature sensitive elements are used throughout the entire abutment screw. A negative temperature (-10 Kelvin, K) is applied to all the nodal points within the abutment screw, causing each element to shrink. A temperature coefficient, C is also required in the analsis and a trial and error process is used to determine equivalent Cs for different levels of F P. In order to replicate F P realisticall, element shrinkage is allowed to occur in the -direction onl. Hence, C is applied in the - direction as well. A summar of T, F P, q and C is presented in Table 2. r n SA 1 Table 2. Summar of loading. r2 r 1 T F P q C 3.5mm C 5.5mm (Nmm) (N) (N/mm 2 ) ( ) ( ) a) Abutment Abutment 3 RESULTS r 3 r t r 4 The von Mises stress distributions in the implant wall are evaluated for all combinations of masticator and preload forces and for two implant sstems with different wall thicknesses. Abutment screw C L SA 2 b) Abutment screw Figure 4. Effective radii of abutment and abutment screw. 3.1 Masticator force, F M The distributions of von Mises stresses on the lines, and for all values of F M are shown in Figure 5. Note that the preload, F P, is set at its medium value, i.e N. Note also that the stress profile and contour for diameter 3.5mm are shown in Figures 5a and b respectivel, and those for 5.5mm are presented in Figures 5c and d respectivel. In general, when the applied masticator force, F M, is increased, the von Mises stresses also increase proportionall, because the sstem being analsed is linear elastic. As epected the 3.5mm implant shows higher stresses than the 5.5mm counterpart (refer to Figures 5a and c). The stress peaks shown in these

5 two figures correspond to the stress concentration areas (i.e. and for 3.5mm and for 5.5mm) as illustrated in Figures 5b and d respectivel. When the wall thisckness is reduced as in the case of the 3.5mm implant, the stress concentration etends from the top of the implant to the first eternal implant thread. van Staden et al. (2007) found that the 4.0 and 4.5mm diameter implants have similar stress distribution characteristics, but lower stress magnitudes at, and when compared to the 3.5mm implant. The 5.5mm implant ields greatl reduced stresses at all locations with stress concentration occurring close to the top of implant onl. a) Stress profile 3.2 Preload, F P To investigate the effect of different preloads F P, F M is kept as a constant at its medium value, i.e. 500N. The distributions of von Mises stresses on the lines, and for all values of F P are shown in Figures 6a and c for 3.5 and 5.5mm implants, respectivel. The corresponding stress contours are presented in Figures 6b and d. Similar stress distribution characteristics are found when varing F P as with F M. Note that when F P increases, the von Mises stresses also increase. However, the increase is not proportional to the increase of F P. This is because F P, as an internal force, is a function of the abutment screw and implant diameters. The large stress variations due to different F M (Figures 5a and c) and the relativel small stress variations due to different F P (Figures 6a and c) demonstrate that F M is more influential than F P to the implant sstem. This suggests that failure of the crown is more likel to be caused b F M. F M = 200N, F M = 500N, F M = 1000N, b) Stress contour 4 CONCLUSION This stud demonstrates that the implant wall thickness significantl influences the stress magnitude and distribution pattern within the implant. Stress concentrations generall occur at the top of the implant as well as around the notch. When the wall thickness is reduced, stress concentrations are also found around the first eternal implant thread. Overall, it is found that the masticator force is more influential on implant stresses than the abutment screw preload. The stud can be further etended to evaluate the impact of the wall thickness in other implant sstems. c) Stress profile 5 ACKNOWLEDGEMENTS A special thank ou goes to Messer John Divitini and Fredrik Engman from Neoss Limited for their continual contribution. F M = 200N, F M = 500N, F M = 1000N, d) Stress contour Figure 5. Stress characteristics when varing F M.

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